Measurement of 131I activity in thyroid using a dual-layer detector

Measurement of 131I activity in the thyroid is vital for monitoring the internal 131I contamination in nuclear accidents or following accidental exposure of the medical staff and public in nuclear medicine services. This measurement is critically influenced by the individualized differences in the thyroid gland, such as the volume depending on age, the difference in its shape, and particularly the overlying tissue thickness (OTT). In this study, a novel method is proposed to estimate the OTT and improve the evaluation of 131I activity in the thyroid using a dual-layer detector. Based on Monte Carlo simulations and calibration experiments, an empirical relationship is established between the 131I activity and the counts at the full-energy peak for both detectors. Using a physical neck-thyroid phantom of Chinese adult males with adjustable OTTs, this method is preliminarily verified at a close neck-detector distance. For different OTTs, the proposed method exhibits an excellent linear response to exposure to 131I in the thyroid. Furthermore, it provides steady estimations of 131I activity in the thyroid with variable OTTs, where the relative standard deviation is less than 5%. The results are expected to provide a new tool for directly monitoring 131I radioactivity in the thyroid gland.